338 EDGAR ALLEN 
the others the function is quite evenly divided between the two 
ovaries. 
5. The period required for the growth of follicles from medium 
size to ovulation size. My material points to an interval of 
surely not more than two cycles (eight to twelve days) and 
probably only one (four to six days) between the beginning of 
the formation of primary liquor folliculi at the poles of the 
medium-sized follicles and the time of their final distention and 
rupture (fig. 20). 
6. Atretic follicles. An observation which seems almost 
universal, although it varies greatly in degree in different ovaries, 
is the presence of atretic follicles of all sizes. Some investiga- 
tors have assumed that ovulation is always spontaneous and 
concluded that follicles atrophy without regard to the stage of 
the oestrous cycle. This conclusion does not necessarily follow 
the finding of atretic follicles of different sizes in the mouse ovary. 
There are two possibilities: first, follicles at any stage of develop- 
ment may atrophy or, second, they may attain full size, but fail 
to rupture at a certain oestrous period, and begin atrophy and 
resorption. The finding of maturation spindles in the ova in 
many of these atretic follicles points to their former large and 
mature state. Continued atrophy may decrease the size until 
there are few granulosa cells present and most of the liquor fol- 
liculi has disappeared. It has been possible to trace sets of 
atretic follicles corresponding in progressive degree of atresia 
to previously recorded oestrous periods through a time equiva- 
lent to three full cycles. In some of these the ova have completed 
their intra-ovarian maturation stages, in others the nuclei are 
in a resting condition. The former points to the follicular 
apparatus, the latter to the ovum itself as the cause of atresia, 
which seems to be by cytolysis of the follicular epithelium. 
b. Corpora lutea. 1. Differences in those of pregnancy and 
oestrus. So much importance has been attributed to the cor- 
pora lutea that a study of those of oestrus has been followed 
with great interest. The time relations of the position of the 
ova in the uterine tubes has been so carefully worked out that 
an estimation of the age of these corpora is quite accurate. 
